HR: 13:55h
AN: T32D-02    [PDF]
TI: Laramide Magmatism in the SW US as a Consequence of Lithospheric Thinning and Thermal Structure Created by Late Jurassic Continental Rifting
AU: * McMillan, N J
EM: nmcmilla@nmsu.edu
AF: New Mexico State University, Department of Geological Sciences, Las Cruces, NM 88003 United States
AU: Lawton, T F
EM: tlawton@nmsu.edu
AF: New Mexico State University, Department of Geological Sciences, Las Cruces, NM 88003 United States
AU: Cowee, C
EM: coweec@hotmail.com
AF: New Mexico State University, Department of Geological Sciences, Las Cruces, NM 88003 United States
AB: Spatial and temporal patterns of Late Cretaceous-Early Cenozoic magmatism in Arizona, New Mexico, west Texas, and northern Mexico demonstrate that arc-like magmatism was prevalent up to 1000 km east of the Farallon-North America trench. Prior to the Late Cretaceous, Mesozoic magmatism was focused in the Sierra Nevada region, with widespread peraluminous intrusions throughout the Cordilleran interior. As early as 80 Ma, however, magmatism shifted far inboard and persisted as late as 50 Ma. Early models for this phenomenon relate an eastward sweep of magmatism to the progressively decreasing subduction angle of the Farallon plate. This model is no longer tenable, for several reasons. 1) Investigation of shallow-angle subduction worldwide demonstrates that magmatism ceases as the subduction angle decreases; modern shallow-angle subduction zones are nearly devoid of active volcanoes. 2) Thermochemical models for melting in subduction zones require both metasomatism of the mantle wedge by slab-derived fluids and convection of the hydrated mantle wedge downward until it reaches its solidus temperature. These processes are minimized or precluded by elimination of the mantle wedge during low-angle subduction. 3) Advances in the quality and quantity of geochronologic data suggest that the locus of magmatism shifted abruptly from near-trench to far inboard, rather than sweeping gradually eastward as previously thought. We propose that Laramide magmatism in the SW US and northern Mexico was not caused by subduction processes, although the igneous rocks have arc-like geochemical characteristics. Instead, we suggest that Laramide magmatism was controlled by the mantle structure inherited from the preceding tectonic event. During Late Jurassic time, southwestern North America experienced extension, resulting in the Border continental rift. The Border rift is defined by thick accumulations of fault-bounded alluvial and marine strata intercalated with rhyolitic ash-flow tuffs and asthenosphere-derived basalts. Border rift basalts have been documented in Kimmeridgian/Tithonian marine and terrigenous strata in the Chiricahua Mountains of SE AZ, in Upper Jurassic strata in the Little Hatchet Mountains of SW NM, and as allochthonous blocks in diapiric Upper Jurassic salts in the La Popa basin of NE Mexico. Because the paleogeographic extent of the Border rift coincides with the region of Laramide magmatism, we interpret Laramide magmatism as the result of dehydration of the Farallon slab and subsequent mantle melting as the slab entered hot asthenospheric mantle emplaced to shallow depths during Border rift extension. Partial melts of metasomatized mantle experienced crustal contamination during ascent, resulting in Laramide igneous rocks with continental arc-like geochemical signatures.
DE: 8110 Continental tectonics--general (0905)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting